A melt extrusion method is a method for obtaining expandable polystyrene resin particles by injecting and kneading an expanding agent into a molten polystyrene resin, in an extruder, extruding the molten resin containing the expanding agent directly into a liquid for cooling through an orifice of a die installed at the front end of the extruder, and cutting the extrusion product with a rapidly rotating blade simultaneously with the extruding, while solidifying the extrusion product by cooling through contact with the liquid.
In the past, in regard to the method for producing expandable polystyrene resin particles by a melt extrusion method, there have been suggested, for example, technologies disclosed in Patent Documents 1 and 2.
Patent Document 1 discloses a method for producing an expanded styrene polymer having a molecular weight Mw of greater than 170,000 g/mole. The method is characterized in that an expanding agent-containing molten styrene polymer product having a temperature of at least 120° C. is conveyed through a die plate having holes with a hole diameter of 1.5 mm or less at the die outlet, and then the extrusion product is granulated.
Patent Document 2 discloses a method for producing thermoplastic expandable resin particles, in which expandable particles are obtained by injecting an expanding agent into a molten thermoplastic resin in an extruder, extruding the molten resin containing the expanding agent directly into a liquid for cooling through a number of orifices of a die installed at the front end of the extruder, cutting the extrusion product with a rapidly rotating blade simultaneously with the extruding, while solidifying the extrusion product by cooling through contact with a liquid. In the method, the extruding is performed such that the shear rate of the expanding agent-containing molten resin upon passing through the orifice land area of the die is 12,000 to 35,000 sec−1, and the apparent melt viscosity of the resin is 100 to 700 poise.
[Patent Document 1] PCT Japanese Translation Patent Publication No. 2005-534733
[Patent Document 2] Pamphlet of WO 2005/028173